Victim cache prefetching

ABSTRACT

A processing unit for a multiprocessor data processing system includes a processor core and a cache hierarchy coupled to the processor core to provide low latency data access. The cache hierarchy includes an upper level cache coupled to the processor core and a lower level victim cache coupled to the upper level cache. In response to a prefetch request of the processor core that misses in the upper level cache, the lower level victim cache determines whether the prefetch request misses in the directory of the lower level victim cache and, if so, allocates a state machine in the lower level victim cache that services the prefetch request by issuing the prefetch request to at least one other processing unit of the multiprocessor data processing system.

This invention was made with United States Government support underAgreement No. HR0011-07-9-0002 awarded by DARPA. The Government hascertain rights in the invention.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates in general to data processing and moreparticularly to data caching in data processing system.

2. Description of the Related Art

A conventional symmetric multiprocessor (SMP) computer system, such as aserver computer system, includes multiple processing units all coupledto a system interconnect, which typically comprises one or more address,data and control buses. Coupled to the system interconnect is a systemmemory, which represents the lowest level of volatile memory in themultiprocessor computer system and generally is accessible for read andwrite access by all processing units. In order to reduce access latencyto instructions and data residing in the system memory, each processingunit is typically further supported by a respective multi-level cachehierarchy, the lower level(s) of which may be shared by one or moreprocessor cores.

Because multiple processor cores may request write access to a samecache line of data and because modified cache lines are not immediatelysynchronized with system memory, the cache hierarchies of multiprocessorcomputer systems typically implement a cache coherency protocol toensure at least a minimum level of coherence among the various processorcore's “views” of the contents of system memory. In particular, cachecoherency requires, at a minimum, that after a processing unit accessesa copy of a memory block and subsequently accesses an updated copy ofthe memory block, the processing unit cannot again access the old copyof the memory block.

A cache coherency protocol typically defines a set of cache statesstored in association with the cache lines stored at each level of thecache hierarchy, as well as a set of coherency messages utilized tocommunicate the cache state information between cache hierarchies. In atypical implementation, the cache state information takes the form ofthe well-known MESI (Modified, Exclusive, Shared, Invalid) protocol or avariant thereof, and the coherency messages indicate a protocol-definedcoherency state transition in the cache hierarchy of the requestorand/or the recipients of a memory access request. The MESI protocolallows a cache line of data to be tagged with one of four states: “M”(Modified), “E” (Exclusive), “S” (Shared), or “I” (Invalid). TheModified state indicates that a memory block is valid only in the cacheholding the Modified memory block and that the memory block is notconsistent with system memory. When a coherency granule is indicated asExclusive, then, of all caches at that level of the memory hierarchy,only that cache holds the memory block. The data of the Exclusive memoryblock is consistent with that of the corresponding location in systemmemory, however. If a memory block is marked as Shared in a cachedirectory, the memory block is resident in the associated cache and inat least one other cache at the same level of the memory hierarchy, andall of the copies of the coherency granule are consistent with systemmemory. Finally, the Invalid state indicates that the data and addresstag associated with a coherency granule are both invalid.

The state to which each memory block (e.g., cache line or sector) is setis dependent upon both a previous state of the data within the cacheline and the type of memory access request received from a requestingdevice (e.g., the processor). Accordingly, maintaining memory coherencyin the system requires that the processors communicate messages via thesystem interconnect indicating their intention to read or write memorylocations. For example, when a processor desires to write data to amemory location, the processor may first inform all other processingelements of its intention to write data to the memory location andreceive permission from all other processing elements to carry out thewrite operation. The permission messages received by the requestingprocessor indicate that all other cached copies of the contents of thememory location have been invalidated, thereby guaranteeing that theother processors will not access their stale local data.

In some systems, the cache hierarchy includes multiple levels, with eachlower level generally having a successively longer access latency. Thus,a level one (L1) cache generally has a lower access latency than a leveltwo (L2) cache, which in turn has a lower access latency than a levelthree (L3) cache.

The level one (L1) or upper-level cache is usually a private cacheassociated with a particular processor core in an MP system. Because ofthe low access latencies of L1 caches, a processor core first attemptsto service memory access requests in its L1 cache. If the requested datais not present in the L1 cache or is not associated with a coherencystate permitting the memory access request to be serviced withoutfurther communication, the processor core then transmits the memoryaccess request to one or more lower-level caches (e.g., level two (L2)or level three (L3) caches) for the requested data.

Typically, when a congruence class of an upper-level cache becomes full,cache lines are removed (“evicted”) and may be written to a lower-levelcache or to system memory for storage. In some cases, a lower levelcache (e.g., an L3 cache) is configured as a “victim” cache, whichconventionally means that the lower level cache is entirely populatedwith cache lines evicted from one or more higher level caches in thecache hierarchy rather than by memory blocks retrieved by an associatedprocessor. Conventional victim caches generally are exclusive, meaningthat a given memory block does not reside in a higher level cache andits associated victim cache simultaneously.

SUMMARY OF THE INVENTION

A processing unit for a multiprocessor data processing system includes aprocessor core and a cache hierarchy coupled to the processor core toprovide low latency data access. The cache hierarchy includes an upperlevel cache coupled to the processor core and a lower level victim cachecoupled to the upper level cache. In response to a prefetch request ofthe processor core that misses in the upper level cache, the lower levelvictim cache determines whether the prefetch request misses in thedirectory of the lower level victim cache and, if so, allocates a statemachine in the lower level victim cache that services the prefetchrequest by issuing the prefetch request to at least one other processingunit of the multiprocessor data processing system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is high level block diagram of an exemplary data processingsystem in accordance with the present invention;

FIG. 2A is a high level block diagram of a processing unit from FIG. 1;

FIG. 2B is a more detailed block diagram of an exemplary embodiment of aprocessor core and associated cache hierarchy from FIG. 2A;

FIG. 2C is an exemplary embodiment of a cache memory from FIG. 2B;

FIG. 3A is a high level logical flowchart of an exemplary method ofperforming a processor load in accordance with one embodiment;

FIG. 3B is a time-space diagram of a sequence of leading and trailingprefetches in accordance with one embodiment;

FIG. 3C is a high level logical flowchart of an exemplary method ofperforming a leading prefetch in accordance with one embodiment;

FIG. 3D is a high level logical flowchart of an exemplary method ofperforming a trailing prefetch for a load or load/store prefetch streamin accordance with one embodiment;

FIG. 3E is a high level logical flowchart of an exemplary method ofperforming a trailing prefetch for a store prefetch stream in accordancewith one embodiment;

FIG. 4 is a high level logical flowchart of an exemplary method ofperforming a processor store in accordance with one embodiment;

FIG. 5 is a high level flowchart of a process for performing an L2eviction and L3 cast-in in accordance with the prior art;

FIG. 6 is a high level flowchart of an exemplary process for performingan L2 eviction and L3 cast-in in accordance with one embodiment.

FIG. 7A is a high level logical flowchart of an L3 eviction inaccordance with one embodiment;

FIG. 7B is a high level logical flowchart of an exemplary process bywhich a snooper handles a castout (CO) command in accordance with oneembodiment; and

FIG. 7C is a high level logical flowchart of the processing of thecoherence responses of a castout (CO) command in accordance with oneembodiment.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENT

With reference now to the figures and, in particular, with reference toFIG. 1, there is illustrated a high level block diagram of an exemplaryembodiment of a multiprocessor data processing system in accordance withthe present invention. As shown, data processing system 100 includesmultiple processing nodes 102 a, 102 b for processing data andinstructions. Processing nodes 102 a, 102 b are coupled to a systeminterconnect 110 for conveying address, data and control information.System interconnect 110 may be implemented, for example, as a busedinterconnect, a switched interconnect or a hybrid interconnect.

In the depicted embodiment, each processing node 102 is realized as amulti-chip module (MCM) containing four processing units 104 a-104 d,each preferably realized as a respective integrated circuit. Theprocessing units 104 a-104 d within each processing node 102 are coupledfor communication by a local interconnect 114, which, like systeminterconnect 110, may be implemented with one or more buses and/orswitches. Local interconnects 114 and system interconnect 110 togetherform an interconnect fabric, which as discussed below preferablysupports concurrent communication of operations of differing broadcastscopes.

The devices coupled to each local interconnect 114 include not onlyprocessing units 104, but also one or more system memories 108 a-108 d.Data and instructions residing in system memories 108 can generally beaccessed and modified by a processor core (FIG. 2A) in any processingunit 104 in any processing node 102 of data processing system 100. Inalternative embodiments of the invention, one or more system memories108 can be coupled to system interconnect 110 rather than a localinterconnect 114.

Those skilled in the art will appreciate that data processing system 100can include many additional unillustrated components, such as peripheraldevices, interconnect bridges, non-volatile storage, ports forconnection to networks or attached devices, etc. Because such additionalcomponents are not necessary for an understanding of the presentinvention, they are not illustrated in FIG. 1 or discussed furtherherein. It should also be understood, however, that the enhancementsprovided by the present invention are applicable to data processingsystems of diverse architectures and are in no way limited to thegeneralized data processing system architecture illustrated in FIG. 1.

Referring now to FIG. 2A, there is depicted a more detailed blockdiagram of an exemplary processing unit 104 in accordance with thepresent invention. In the depicted embodiment, each processing unit 104includes multiple instances of a processor core and associated cachehierarchy, which are collectively identified by reference numeral 200.In the depicted embodiment, each processing unit 104 also includes anintegrated memory controller (IMC) 206 that controls read and writeaccess to one or more of the system memories 108 a-108 d within itsprocessing node 102 in response to requests received from processorcores and operations snooped on the local interconnect 114.

Still referring to FIG. 2A, each processing unit 104 also includes aninstance of coherence management logic 210, which implements a portionof the distributed snoop-based coherency signaling mechanism thatmaintains cache coherency within data processing system 100. Inaddition, each processing unit 104 includes an instance of forwardinglogic 212 for selectively forwarding communications between its localinterconnect 114 and system interconnect 110. Finally, each processingunit 104 includes an integrated I/O (input/output) controller 214supporting the attachment of one or more I/O devices, such as I/O device216. I/O controller 214 may issue operations on local interconnect 114and/or system interconnect 110 in response to requests by I/O device216.

With reference now to FIG. 2B is a more detailed block diagram of anexemplary embodiment of a processor core and associated cache hierarchy200 from FIG. 2A. Processor core 202 includes circuitry for processinginstructions and data. In the course of such processing, the circuitryof processor core 202 generates various memory access requests, such asload and store requests.

The operation of processor core 202 is supported by a cache memoryhierarchy including a store-through level one (L1) cache 204 within eachprocessor core 202, a store-in level two (L2) cache 230, and a lookasideL3 cache 232 that is utilized as a victim cache for L2 cache 230 andaccordingly is filled by cache lines evicted from L2 cache 230. Incontrast to many conventional victim cache arrangements, the contents ofL3 cache 232 are not exclusive of the contents of L2 cache 230, meaningthat a given memory block may be held concurrently in L2 cache 230 andL3 cache 232.

In at least some embodiments, processor core 202 further includes astreaming prefetcher 203 that generates and transmits to the memoryhierarchy prefetch requests requesting data to be staged into its cachememory hierarchy in advance of need (e.g., prior to a demand load orstore). In preferred embodiments, streaming prefetcher 203 supportsmultiple concurrent prefetching streams, and in at least some cases,supports multiple concurrent prefetching stream types having differingbehaviors. For example, in one exemplary embodiment, streamingprefetcher 203 includes a load prefetch stream to prefetch memory blocksthat may be the target of load requests, a store prefetch stream toprefetch memory blocks that may be targets of store requests, and aload/store prefetch stream to prefetch memory blocks that may be targetof load and/or store requests. These different prefetch streams may havedifferent associated strides, stream depths, caching rules, etc., asdiscussed further below. In other embodiments, processor core 202 mayimplement prefetching without streaming, that is, without fetching froma sequence of addresses linked by a common stride.

In order to support prefetching while limiting the associated cost andlatency impact on the cache memory hierarchy, L3 cache 232 includes atleast one and preferably many prefetch machines (PFMs) 234 a-234 n that,in response to prefetch requests issued by streaming prefetcher 203 thatmiss in the cache memory hierarchy, manage the transmission of theprefetch requests to the system for service and the installation ofprefetch data in the cache memory hierarchy, as discussed further belowwith reference to FIGS. 3B-3E. In one embodiment, prefetch machines 234a-234 n can be implemented within master 284 (see FIG. 3) asspecial-purpose prefetch machines dedicated to handling prefetchrequests, as disclosed in greater detail in U.S. patent application Ser.No. 11/457,333, which was filed Jul. 13, 2006, and is incorporatedherein by reference in its entirety.

FIG. 2B also illustrates an exemplary flow of requests, data andcoherence communication within the cache memory hierarchy of processorcore 202. In the depicted arrangement, dashed lines represent the flowof requests and coherence commands, and solid lines represent data flow.

As shown, processor core 202 transmits load requests 240 to, andreceives load data 242 from L2 cache 230. Processor core 202 alsotransmits store requests 244 and associated store data 246 to gatheringlogic 248, which gathers the store data associated with multiplerequests into one cache line of data and transmits the gathered storedata 249 to L2 cache 230 in conjunction with one gathered store request247. Although illustrated separately for clarity, gathering logic 248may be incorporated within processor core 202 and/or L2 cache 230.

L2 cache 230 transmits system coherence commands 250 to coherencemanagement logic 210 of FIG. 2A for compilation and/or transmission onthe interconnect fabric. L2 cache 230 also transmits write data 254 to,and receives load data 252 from IMC 206 and/or interconnect logic 212.L2 cache 230 may also request load data from L3 cache 232 via a loadrequest 260 and receive load data 262 from L3 cache 232. To remove acache line from L2 cache 230, L2 cache 230 may issue a cast-in requestto L3 cache 232, which in turn receives the cache line as cast-in data266. Similar to L2 cache 230, L3 cache 232 may interact with IMCs 206and/or cache memories in other cache hierarchies by issuing systemcoherence commands 270, receiving prefetch data 272, and transmittingwrite data 274.

Although the illustrated cache hierarchy includes only three levels ofcache, those skilled in the art will appreciate that alternativeembodiments may include additional levels (L4, L5, etc.) of on-chip oroff-chip in-line or lookaside cache, which may be fully inclusive,partially inclusive, or non-inclusive of the contents the upper levelsof cache. Further, any of the various levels of the cache hierarchy maybe private to a particular processor core 202 or shared by multipleprocessor cores 202. For example, in some implementations, the cachehierarchy includes an L2 cache 230 for each processor core 202, withmultiple of the L2 caches 230 sharing a common L3 victim cache 232.

Referring now to FIG. 2C, there is depicted an exemplary embodiment of acache memory 280 that may be utilized to implement L2 cache 230 or L3cache 232 from FIG. 2B. As shown, cache memory 280 includes an array anddirectory 282, as well as a cache controller comprising a master 284 anda snooper 286. Snooper 286 snoops operations from local interconnect114, provides appropriate responses, and performs any accesses to arrayand directory 282 required by the operations. Master 284 initiatestransactions on local interconnect 114 and system interconnect 110 andaccesses array and directory 282 in response to memory access (andother) requests originating within the processor core and cachehierarchy 200. In at least some embodiments, master 284 also handlescasting out data to lower levels of the memory hierarchy (e.g., L3victim cache 232 or system memory 108).

Array and directory 282 includes a set associative cache array 284including multiple ways 286 a-286 n. Each way 286 includes multipleentries 288, which in the depicted embodiment each provide temporarystorage for up to a full memory block of data, e.g., 128 bytes. Eachcache line or memory block of data is logically formed of multiplesub-blocks 290 (in this example, four sub-blocks of 32 bytes each) thatmay correspond in size, for example, to the smallest allowable access tosystem memories 108 a-108 d. In at least some embodiments, sub-blocks290 may be individually accessed and cached in cache array 284.

Array and directory 282 also includes a cache directory 292 of thecontents of cache array 284. As in conventional set associative caches,memory locations in system memories 108 are mapped to particularcongruence classes within cache arrays 284 utilizing predetermined indexbits within the system memory (real) addresses. The particular cachelines stored within cache array 284 are recorded in cache directory 292,which contains one directory entry for each cache line in cache array284. As understood by those skilled in the art, each directory entry incache directory 292 comprises at least a tag field 294, which specifiesthe particular cache line stored in cache array 284 utilizing a tagportion of the corresponding real address, a LRU (Least Recently Used)field 298 indicating a replacement order for the cache line with respectto other cache lines in the same congruence class, and a state field296, which indicates the coherence state (also referred to as cachestate) of the cache line.

Although the exemplary embodiment illustrates that each state field 296provides state information for a respective associated cache line incache array 284, those skilled in the art will appreciate that inalternative embodiments a cache directory 292 can include a respectivestate field for each sub-block 290. Regardless of which implementationis selected, the quantum of data associated with a coherence state isreferred to herein as a coherence granule.

To support the transfer of castout cache lines, array and directory 282includes at least one and preferably multiple castout (CO) buffers 295a-295 n, which are each preferably identified with a unique respectiveCO buffer ID. While a CO buffer 295 is allocated to master 284 for acastout operation, the CO buffer 295 has a “busy” state, and when the CObuffer is released or deallocated by master 284, then the CO 295 bufferhas a “done” state.

In a preferred embodiment, data processing system 100 maintainscoherency with a non-blocking, broadcast-based coherence protocol thatutilizes a set of predefined coherence states in state fields 296 and arobust set of associated request, response, and notification types.Coherence requests are broadcast with a selected scope to cachememories, as well as IMCs 206 and I/O controllers 214. As discussedfurther below, the selected scope of broadcast can be “global”, that is,inclusive of all participants (e.g., IMCs 206, IOCs 214, L2 caches 230and L3 caches 232) in data processing system 100 or have a morerestricted scope excluding at least some participants. In response tosnooping the coherence requests, the participants provide partialresponses (PRESPs), which are aggregated (preferably at coherencemanagement logic 210 of the requesting processing unit 104) to form thebasis for a coherence transfer decision. Notification of the decision issubsequently broadcast to the participants in a combined response(CRESP) indicating the final action to be taken. Thus, the coherenceprotocol employs distributed management.

In a preferred embodiment, global and local (or scope-limited) broadcasttransport mechanisms are both integrated. Thus, a given request can bebroadcast globally or locally, where a local scope may correspond, forexample, to a single processing node 102. If all information necessaryto resolve a coherence request exists within the local broadcast scope,then no global broadcast is necessary. If a determination cannot be madethat all information necessary to resolve the coherence request ispresent within the local broadcast scope, the coherence request isbroadcast globally (or at least with an increased scope including atleast one additional participant).

To ensure a reasonable likelihood of a successful local resolution ofcoherence requests, a mechanism indicative of the distribution of cachedcopies of memory blocks within the cache hierarchies is useful. In apreferred embodiment, the mechanism includes inclusion of a scope-stateindication per memory block (e.g., 128 bytes) in system memory 108 andan appropriate set of coherence states for state fields 296 in L2 and L3caches 230, 232. In one embodiment, the scope-state indication for eachmemory block is a single bit integrated into the redundant content forerror correction stored in system memory 108. For each memory block, thescope-state indicator indicates whether the memory block might be in useoutside of the local scope where the system memory 108 resides. Sincethe scope-state indicator is stored with the data bits, the scope-statebit is automatically read or written whenever the data is read orwritten.

Coherence states that may be utilized in state field 296 to indicatestate information may include those set forth in Table I below. Table Ilists the name of various coherence states in association with adescription of the state, an indication of the authority conveyed by thecoherence state to read and/or update (which includes the authority toread) the associated cache line, an indication of whether the coherencestate permits other cache hierarchies to concurrent hold the associatedcache line, an indication of whether the associated cache line iscastout upon deallocation, and an indication of if and when theassociated cache line is to be sourced in response to snooping a requestfor the cache line. A further description of the implementation of atleast some of these coherence states is described in detail in U.S.patent application Ser. No. 11/055,305, which is incorporated herein byreference.

TABLE I State Description Authority Sharers Data Castout Source data IInvalid None N/A N/A N/A Id Deleted, do not allocate None N/A N/A N/A IgInvalid, cached scope-state None N/A N/A N/A In Invalid, scope predictorNone N/A N/A N/A S Shared Read Yes No No Sl Shared, local data sourceRead Yes No At request T Formerly MU, now shared Update Yes Yes At CRESPTe Formerly ME, now shared Update Yes No At CRESP Tn Formerly MU, nowshared Update Yes Yes At CRESP Ten Formerly ME, now shared Update Yes NoAt CRESP M Modified, avoid sharing Update No Yes At request Me ExclusiveUpdate No No At request Mu Modified, bias toward sharing Update No YesAt request

As shown in Table II below, a number of the coherence states set forthin Table I provide low-latency access to high-usage scope states whileprotecting system memories 108 from increased traffic due to scope-statequeries and updates. Note that when a cached scope state is deallocated,it is typically cast out (i.e., written back) to memory. For cases inwhich the implied scope state might be global, the castout isfunctionally required to ensure that coherence is maintained. For casesin which the implied scope state is known to be local, the castout isoptional, as it is desirable but not necessary to localize the broadcastscope for subsequent operations.

TABLE II State Implied scope state Scope-state castout I None None IdNone None Ig Existing copies probably global Required, global InExisting copies probably local None S Unknown None Sl Unknown None TShared copies probably global Required, global Te Shared copies probablyglobal Required, global Tn Shared copies all local Optional, local TenShared copies all local None M Local Optional, local Me Local None MuLocal Optional, local

The combination of the scope-state bits in system memory 108 and thecoherence states described herein provides a low-cost alternative to adirectory-based approach and integrates cleanly into the non-blocking,broadcast-based distributed coherence protocol. Because some workloadslocalize well and others do not, processing unit 104 may alsoincorporate a number of predictors to determine whether a givencoherence request should be initially broadcast with a local scope orshould be broadcast globally immediately. For workloads that exhibit ahigh degree of processor-to-memory localization, and for workloads thathave varying mixtures of locally resolvable traffic, laboratory resultsshow that scope-limited speculative snoop resolution is highlyeffective.

With reference now to FIG. 3A, there is illustrated a high level logicalflowchart of an exemplary method of performing a load of a processorcore in accordance with one embodiment. The illustrated process beginsat block 300 in response to receipt by L2 cache 230 of a load request240 from its associated processor core 202 following a miss in the L1cache 204. In response to the load request 240, master 284 of L2 cache230 accesses its cache directory 292 to determine whether or not thetarget address specified by load request 240 hits in cache directory 292(block 302). If so, the process then proceeds to blocks 304 and 306,which depict master 284 of L2 cache 230 reading the requested cache lineof data from its cache array 284 and then sending the cache line of datato the requesting processor core 202. Thereafter, the process terminatesat block 326.

Returning to block 302, in response to an L2 miss, the process proceedsto block 310, which illustrates L2 cache 230 selecting and initiatingeviction of a victim cache line, as discussed further below withreference to FIGS. 5-6. In addition, L2 cache 230 transmits the loadrequest to L3 cache 232 as a load request 260. Consequently, master 284of L3 cache 232 accesses its cache directory 292 to determine whether ornot the target address specified by load request 260 hits in cachedirectory 292 of L3 cache 232 (block 312). If not, the process passes toblock 320, which is described below. If, however, load request 260 hitsin cache directory 292 of L3 cache 232, the process proceeds to block314, which depict master 284 of L3 cache 232 reading the requested cacheline of data from cache array 284 of L3 cache 232 and providing therequested cache line to L2 cache 230. The process then bifurcates andproceeds to blocks 306 and 316.

As noted above, block 306 depicts L3 cache 232 sending the requestedcache line of data to the requesting processor core 202. Thereafter, thefirst branch of the process ends at block 326. Block 316 illustratesmaster 284 of L3 cache 232 updating the coherence state of the requestedcache line of data in cache directory 292 of L3 cache 232 in accordancewith Table III, below.

TABLE III Final L3 State Initial L3 Prefetch Load Prefetch PrefetchState Load Ifetch Xlate (Load) (Lock) (Store) (Ld/St) M SL, LRU SL, LRUSL, LRU SL, LRU I I I Mu SL, LRU SL, LRU SL, LRU SL, LRU I I I Me SL,LRU SL, LRU SL, LRU SL, LRU I I I T S, LRU S, LRU S, LRU S, LRU S, LRUS, LRU S, LRU Te S, LRU S, LRU S, LRU S, LRU S, LRU S, LRU S, LRU Tn S,LRU S, LRU S, LRU S, LRU S, LRU S, LRU S, LRU Ten S, LRU S, LRU S, LRUS, LRU S, LRU S, LRU S, LRU SL S, LRU S, LRU S, LRU S, LRU S, LRU S, LRUS, LRU S S, LRU S, LRU S, LRU S, LRU S, LRU S, LRU S, LRU Ig (n/a) (n/a)(n/a) (n/a) (n/a) (n/a) (n/a) In (n/a) (n/a) (n/a) (n/a) (n/a) (n/a)(n/a) I (n/a) (n/a) (n/a) (n/a) (n/a) (n/a) (n/a)

In contrast with conventional implementations in which any fetch thathit in an L3 victim cache in a data-valid coherency state (e.g., M, Mu,Me, T, Te, Tn, Ten, Sl or S) resulted in the invalidation of thematching cache line in the L3 directory, Table III discloses that afetch hit in the Tx or Sx states (where the “x” refers to any variant ofthe base coherence state) preserves the matching cache line in L3 cache232 in the S state and in a replacement order other than Most RecentlyUsed (e.g., LRU or LRU−1). In this way, the likelihood of a castout hitin L3 cache 232 is increased, which as discussed further below, reducesdata movement and thus power dissipation in the event of an L2 eviction.

Further, for a hit in an Mx (e.g., M, Mu or Me) state, the coherencystate is updated to either SL or I, depending upon the type of memoryaccess requested. For core loads, as depicted in FIG. 3A, as well as forinstruction fetches (Ifetch), fetches of page table entries containinginformation utilized for address translation (Xlate), and prefetches forload prefetch streams, the matching entry is preferably updated with acoherency state of SL and a replacement order other than Most RecentlyUsed (e.g., LRU or LRU−1). Atomic loads, prefetches generated within astore prefetch stream and prefetches generated within a load/storeprefetch stream preferably cause the matching entry is to be invalidated(i.e., set to I). The distinction in the final L3 cache states is madebased upon different expectations as to whether a store to the memoryblock will subsequently be made. For instruction fetches, fetches ofpage table entries, and prefetches for load prefetch streams, no storeoperation is likely. Thus, it is helpful if the target memory block isretained in L3 cache 232. However, for atomic loads, prefetchesgenerated within a store prefetch stream and prefetches generated withina load/store prefetch stream, a subsequent store to the target memoryblock is extremely likely, and leaving a copy of the memory block in L3cache 232 would require a background kill bus operation to invalidatethe L3 copy when a subsequent store to the memory block is made. Theadditional background kill bus operation would not only dissipateadditional power, but also prolong the duration of the store operationmust be managed by master 284 of L2 230.

As illustrated at block 318, master 284 of L2 cache 230 also updates thestate of the requested cache line of data in cache directory 292 of L2cache 230 in accordance with Table IV, below. In the depicted exemplaryembodiment, the coherency state is updated in cache directory 292 of L2cache 230 to the initial state of the cache line in L3 cache 232 if theinitial coherence state of the target memory block in cache directory292 of L3 cache 232 is other than Mx (e.g., M, Mu or Me). For coreloads, as depicted in FIG. 3A, as well as for instruction fetches(Ifetch), fetches of page table entries containing information utilizedto perform address translation (Xlate), and prefetches for load prefetchstreams, the matching entry is preferably updated in L2 cache 230 to Tnif the initial state in L3 cache 232 is M or Mu, and is updated to Tenin L2 cache 230 if the initial state in L3 cache 232 is Me. An L2coherence state with less authority than the initial L3 coherence stateis employed for these types of memory access requests because of the lowlikelihood of a subsequent store and the desire to avoid data movementin the event of a subsequent L2 castout. However, it is preferable if L2cache 230 is updated to the initial coherence state in L3 cache 232 ifthe requested memory access is an atomic load, prefetch generated withina store prefetch stream, or prefetch generated within a load/storeprefetch stream that hits in L3 cache 232 in an Mx coherence statebecause of the high likelihood that these operations will be followed bya store operation.

TABLE IV Final L2 State Initial L3 Prefetch Load Prefetch Prefetch StateLoad Ifetch Xlate (load) (Lock) (Store) (Ld/St) M Tn Tn Tn Tn M M M MuTn Tn Tn Tn Mu Mu Mu Me Ten Ten Ten Ten Me Me Me T T T T T T T T Te TeTe Te Te Te Te Te Tn Tn Tn Tn Tn Tn Tn Tn Ten Ten Ten Ten Ten Ten TenTen SL SL SL SL SL SL SL SL S S S S S S S S Ig (n/a) (n/a) (n/a) (n/a)(n/a) (n/a) (n/a) In (n/a) (n/a) (n/a) (n/a) (n/a) (n/a) (n/a) I (n/a)(n/a) (n/a) (n/a) (n/a) (n/a) (n/a)

As shown at block 324, once the victim cache line has been evicted fromL2 cache 230, the cache line of data supplied to processor core 202 isalso installed in L2 cache 230 (block 324). Thereafter, the processterminates at block 326.

Referring now to block 320, in response to the load requests 240, 260missing in L2 cache 230 and L3 cache 232, master 284 of L2 cache 230requests access authority and the target memory block from the systemcoherence manager (e.g., the distributed coherence management systemdescribed above) by transmitting an appropriate command 250 to the localinstance of interconnect logic 212. Master 284 then updates thecoherence state for the target memory block in its cache directory 292in accordance with the coherence response (also referred to as combinedresponse (CRESP)) for its request (block 322). Master 284 also suppliesthe target memory block to the requesting processor core, as indicatedby the process passing through page connector A to block 306. Inaddition, once eviction of the L2 victim is complete and load data 252is received, master 284 updates cache array 284 with the target memoryblock (block 324). Thereafter, the process ends at block 326.

With reference now to FIG. 3B, there is depicted a time-space diagram ofan exemplary prefetching sequence in accordance with one embodiment. Inthe diagram, a particular processor core and cache hierarchy 200 isdepicted as containing an L1 cache 204, L2 cache 230 and L3 cache 232,and the remainder of data processing system 100 is collectivelyrepresented as system 328.

In the depicted exemplary prefetching sequence, a stream of leadingprefetch (PF) requests 330 is generated by the streaming prefetcher 203in the processor core 202 and then passed to the cache memory hierarchy.Thus, in contrast to demand load requests, the leading prefetch requests(as well as other prefetch requests) are not generated through theexecution of an ISA instruction by the instruction execution circuitryof processor core 202, but rather generated by streaming prefetcher 203in anticipation of execution of one or more ISA instructions thatimplicitly or explicitly indicate a memory access. Although the leadingprefetch requests 330 accesses each level of the cache memory hierarchy,as shown in FIG. 3B by circles, it is generally the case that the targetmemory blocks of leading prefetch requests 330 do not initially residein the cache memory hierarchy. Accordingly, a prefetch machine (PFM) 234within L3 cache 232 generally issues leading prefetch requests 330 tosystem 328, which supplies the target memory blocks of leading prefetchrequests as prefetch data 332. In contrast to conventional prefetchingschemes, prefetch data 332 responsive to leading prefetch requests 330are installed in L3 (victim) cache 232 rather than directly in L2 cache230.

In some operating scenarios, for purposes of local optimization, leadingprefetch requests 330 are discarded at some level of the cache memoryhierarchy and not forwarded to a lower level of the cache memoryhierarchy or system 328. Because leading prefetch requests 330 arespeculative in nature and are generated to reduce latency rather than inresponse to a demand memory access, the discarding of a leading prefetchrequest will not affect correctness.

Subsequent to a leading prefetch request 330 and nearer in time to ananticipated demand memory access request (e.g., demand load or storerequest), streaming prefetcher 203 issues a corresponding trailingprefetch request 334 targeting the same target memory block. Althoughtrailing prefetch requests 334 access each level of the cache memoryhierarchy, as shown in FIG. 3B by circles, it is generally the case thatthe target memory block of a trailing prefetch request 334 initiallyresides only in L3 cache memory 232 as a result of the earliercorresponding leading prefetch request 330. Accordingly, L3 cache 232generally services a trailing prefetch request 334 by supplying thetarget memory block of the trailing prefetch request to one or morehigher levels of cache memory as prefetch data 336. For example, in anembodiment described below, prefetch data 336 of load or load/storeprefetch streams are installed in both L1 cache 204 and L2 cache 230,while prefetch data 336 of store prefetch streams are installed in L2cache 230, but not L1 cache 204. This distinction is made because in thepreferred embodiment, L1 cache 230 is a store-through cache and L2 cache230 is a store-in cache, meaning that all store requests are resolved atL2 cache 230.

With the prefetch data staged within the cache memory hierarchy in themanner described above, a demand memory access 338 (e.g., a demand loador store request) subsequent to a leading prefetch request 330 and atrailing prefetch request 334 is serviced with an optimal accesslatency.

Referring now to FIG. 3C, there is depicted a high level logicalflowchart of an exemplary method of performing a leading prefetch inaccordance with one embodiment. The illustrated process begins at block340 following a miss of leading prefetch request in the L1 cache 204 andthen proceeds to block 342, which depicts a determination by L2 cache230 whether or not the leading prefetch request hits in cache directory292 of L2 cache 230. If so, the leading prefetch request is aborted, andthe process terminates at block 349. If, however, the leading prefetchrequest misses in L2 cache 230, the process proceeds to block 344.

Block 344 depicts a determination by L3 cache 232 whether or not theleading prefetch request hits in cache directory 292 of L3 cache 232. Ifso, the leading prefetch request is aborted, and the process terminatesat block 349. If, however, the leading prefetch request misses in L3cache 232, the process proceeds to block 345. Block 345 illustrates L3cache 232 allocating a prefetch machine 234 to manage the leadingprefetch request, which in turn initiates the process of evicting avictim entry from L3 cache 232 in preparation for receiving the prefetchdata requested by the leading prefetch request.

Next, at block 346, the prefetch machine 234 allocated to the leadingprefetch request requests access authority and the target memory blockfrom the system coherence manager (e.g., the distributed coherencemanagement system described above) by transmitting an appropriatecommand 250 to the local instance of interconnect logic 212. Prefetchmachine 234 then updates the coherence state for the target memory blockin its cache directory 292 in accordance with the coherence response(also referred to as combined response (CRESP)) for its request (block347). In addition, once eviction of the L3 victim entry is complete andprefetch data 332 is received, prefetch machine 234 updates cache array284 of L3 cache 232 with the target memory block (block 348).Thereafter, the process ends at block 349.

With reference now to FIG. 3D, there is illustrated a high level logicalflowchart of an exemplary method of performing a trailing prefetch for aload or load/store prefetch stream in accordance with one embodiment.The process depicted in FIG. 3D begins at block 350 following a miss oftrailing prefetch request of a load or load/store prefetch stream in anL1 cache 204 and then proceeds to block 352. At block 352, master 284 ofL2 cache 230 accesses its cache directory 292 to determine whether ornot the target address specified by the trailing prefetch request hitsin cache directory 292 of L2 cache 230. If so, the process then proceedsto blocks 354 and 356, which depict master 284 of L2 cache 230 readingthe requested cache line of data from its cache array 284 and thensending the target cache line of data to the requesting processor core202. Thereafter, the process terminates at block 376.

Returning to block 352, in response to an L2 miss, the process proceedsto block 360, which illustrates L2 cache 230 selecting and initiatingeviction of a victim cache line, as discussed further below withreference to FIGS. 5-6. In addition, L2 cache 230 transmits the trailingprefetch request to L3 cache 232. Consequently, L3 cache 232 accessesits cache directory 292 to determine whether or not the target addressspecified by the trailing prefetch request hits in cache directory 292of L3 cache 232 (block 362). If not, the process passes to block 363,which is described below. If, however, the trailing prefetch requesthits in cache directory 292 of L3 cache 232, the process proceeds toblock 364, which depicts L3 cache 232 reading the requested cache lineof data from cache array 284 of L3 cache 232 and providing the requestedcache line to L2 cache 230. The process then bifurcates and proceeds toblocks 356 and 366.

As noted above, block 356 depicts L3 cache 232 sending the requestedcache line of data to the requesting processor core 202. Thereafter, thefirst branch of the process ends at block 376. Block 366 illustrates L3cache 232 updating the coherence state of the requested cache line ofdata in cache directory 292 of L3 cache 232 in accordance with TableIII, above. As illustrated at block 368, master 284 of L2 cache 230 alsoupdates the state of the requested cache line of data in cache directory292 of L2 cache 230, if necessary, in accordance with Table IV, above.As shown at block 374, once the victim cache line has been evicted fromL2 cache 230, the cache line of data supplied to processor core 202 isalso installed in L2 cache 230 (block 374). Thereafter, the processterminates at block 376.

Referring now to block 363, if a trailing prefetch request misses in L3cache 232, master 284 within L2 cache 230 does not immediately transmitthe trailing prefetch request to the broader system for service.Instead, at block 363 master 284 first checks whether the trailingprefetch request collides (i.e., has a matching target address) withanother memory access request currently being serviced by master 284 ofL3 cache 232 (i.e., a leading prefetch request being handled by aprefetch machine 234). If not, the process passes directly to block 370,which is described below. If, however, the trailing prefetch requestcollides with another memory access request currently being serviced bymaster 284 of L3 cache 232, then master 284 of L2 cache 230 waits untilthe other memory access request is resolved, as shown at block 365, andthereafter again checks whether the trailing memory access request hitsin cache directory 292 of L3 cache 232, as shown at block 362 and asdescribed above. In this manner, bandwidth on the system interconnectsis not unnecessarily consumed by the address and data tenures ofprefetch requests, which are necessarily speculative.

Referring now to block 370, master 284 of L2 cache 230 requests accessauthority and the target memory block from the system coherence manager(e.g., the distributed coherence management system described above) bytransmitting an appropriate command 250 to the local instance ofinterconnect logic 212. In response to receipt of the coherence response(also referred to as combined response (CRESP)) and prefetch data forthe trailing prefetch request, master 284 of L2 cache 230 updates thecoherence state for the target memory block in its cache directory 292in accordance with the coherence response (block 372). Master 284 of L2cache 230 also supplies the target memory block to the requestingprocessor core 202, as indicated by the process passing through pageconnector A to block 306. In addition, once eviction of the L2 victim iscomplete and the prefetch data is received, master 284 of L2 cache 230updates the cache array 284 of L2 cache 230 with the target memory block(block 374). Thereafter, the process ends at block 376.

It should be noted that in the case of a miss of a trailing prefetch inL3 cache 232, the prefetch data is not installed in L3 cache 232. L3cache 232 is “skipped” for purposes of data installation because, inmost cases, a subsequent demand memory access will be serviced by ahigher level of the cache memory hierarchy.

Referring now to FIG. 3E, there is depicted a high level logicalflowchart of an exemplary method of performing a trailing prefetch for astore prefetch stream in accordance with one embodiment. The processdepicted in FIG. 3E begins at block 380 following receipt at an L2 cache230 of a trailing prefetch request of a store prefetch stream from theassociated processor core 202. The process then proceeds to block 381,which illustrates master 284 of L2 cache 230 accessing its cachedirectory 292 to determine whether or not the target address specifiedby the trailing prefetch request hits in cache directory 292 of L2 cache230. If so, the target memory block is already staged to store-in L2cache 230, meaning that no prefetching is required. Accordingly, theprocess terminates at block 392.

Returning to block 381, in response to an L2 miss, the process proceedsto block 382, which illustrates L2 cache 230 selecting and initiatingeviction of a victim cache line, as discussed further below withreference to FIGS. 5-6. In addition, L2 cache 230 transmits the trailingprefetch request to L3 cache 232. Consequently, L3 cache 232 accessesits cache directory 292 to determine whether or not the target addressspecified by the trailing prefetch request hits in cache directory 292of L3 cache 232 (block 383). If not, the process passes to block 387,which is described below. If, however, the trailing prefetch requesthits in cache directory 292 of L3 cache 232, the process proceeds toblock 384, which depicts L3 cache 232 reading the requested cache lineof data from cache array 284 of L3 cache 232 and sending the requestedcache line of data to L2 cache 230. The process then proceeds to block385.

Block 385 illustrates L3 cache 232 updating the coherence state of therequested cache line of data in cache directory 292 of L3 cache 232 inaccordance with Table III, above. Master 284 of L2 cache 230 alsoupdates the state of the requested cache line of data in cache directory292 of L2 cache 230 in accordance with Table IV, above (block 386). Asshown at block 391, once the victim cache line has been evicted from L2cache 230, the cache line of prefetch data is installed in L2 cache 230(block 391). Thereafter, the process terminates at block 392.

Referring now to block 387, if a trailing prefetch request misses in L3cache 232, master 284 of L2 cache 230 does not immediately transmit thetrailing prefetch request to the broader system for service. Instead, atblock 387 master 284 of L2 cache 230 first checks whether the trailingprefetch request collides (i.e., has a matching target address) withanother memory access request currently being serviced by master 284 ofL3 cache 232 (i.e., a leading prefetch request being handled by aprefetch machine 234). If not, the process passes directly to block 389,which is described below. If, however, the trailing prefetch requestcollides with another memory access request currently being serviced bymaster 284 of L3 cache 232, then master 284 of L2 cache 230 waits untilthe other memory access request is resolved, as shown at block 388, andthereafter again checks whether the trailing memory access request hitsin cache directory 292 of L3 cache 232, as shown at block 383 and asdescribed above. In this manner, bandwidth on the system interconnectsis not unnecessarily consumed by the address and data tenures ofprefetch requests.

Referring now to block 389, master 284 of L2 cache 230 requests accessauthority and the target memory block from the system coherence manager(e.g., the distributed coherence management system described above) bytransmitting an appropriate command 250 to the local instance ofinterconnect logic 212. In response to receipt of the coherence responseand prefetch data for the trailing prefetch request, master 284 of L2cache 230 updates the coherence state for the target memory block in itscache directory 292 in accordance with the coherence response (block390). In addition, once eviction of the L2 victim is complete and theprefetch data is received, master 284 of L2 cache 230 updates the cachearray 284 of L2 cache 230 with the target memory block of the trailingprefetch request (block 391). Thereafter, the process ends at block 392.

Referring now to FIG. 4, there is depicted a high level logicalflowchart of an exemplary method of performing a processor store inaccordance with one embodiment. The illustrated process begins at block400 in response to receipt by L2 cache 230 of a store request 247 fromits associated processor core 202. In response to store request 247,master 284 of L2 cache 230 accesses its cache directory 292 to determinewhether or not the target address specified by load request 240 hits incache directory 292 (block 402). If so, the process then proceeds toblock 404, which depicts master 284 of L2 cache 230 reading therequested cache line of data from cache array 284 of L2 cache 230. Theprocess then passes to block 406, which is described below.

Returning to block 402, in response to a determination that the targetaddress of the store request 247 missed in cache directory 292 of L2cache 230, master 284 initiates eviction of a victim cache line from L2cache 230, as shown at block 410 and as described further below withreference to FIGS. 5-6. Master 284 also forwards the target address ofstore request 247 to L3 cache 232. In response to receipt of the targetaddress of store request 247, master 284 of L3 cache 232 accesses itscache directory 292 to determine whether or not the target addressspecified by load request 240 hits in cache directory 292 (block 420).If not, the process passes to block 424 and following blocks, which aredescribed below. If, however, the target address of store request 247hits in cache directory 292 of L3 cache 232, the process proceeds toblock 422, which depicts master 284 of L3 cache 232 reading therequested cache line of data from cache array 284 of L3 cache 232. Theprocess then passes to block 406.

Block 406 determines the master 284 of the L2 or L3 cache memory inwhich the target address hit determining whether or not it is thehighest point of coherency (HPC) for the target memory block associatedwith the target address. An HPC is defined herein as a uniquelyidentified device that caches a true image of the memory block (whichmay or may not be consistent with the corresponding memory block insystem memory 108) and has the authority to grant or deny a request tomodify the memory block. Descriptively, the HPC may also provide a copyof the memory block to a requester in response to an operation that doesnot modify the memory block. Although other indicators may be utilizedto designate an HPC for a memory block, a preferred embodiment of thepresent invention designates the HPC, if any, for a memory blockutilizing selected cache coherence state(s). Thus, assuming thecoherence states set forth in Tables I and II, above, an L2 cache 230 orL3 cache 232 is designated as an HPC by holding the target memory blockin any of the T, Te, Tn, Ten, M, Me or Mu states.

If the master 284 determines at block 406 that its cache 230 or 232 isthe HPC for the target memory block, the process passes to block 412,which is described below. If, however, the master 284 determines thatits cache is not the HPC for the target memory block, for example,because the target address hit in the S or Sl coherence state, thenmaster 284 attempts to claim coherence ownership of the target memoryblock and assume the designation of HPC by transmitting a DClaim (dataclaim) operation on the interconnect fabric via interconnect logic 212(block 408). Master 284 determines whether the attempt to claimcoherence ownership is granted at block 410 by reference to the systemcoherence response (CRESP) to the DClaim. If the attempt to claimcoherence ownership is not granted, which typically means that master284 has been forced to invalidate its copy of the target memory block bya competing master 284 in another cache hierarchy, the process passesthrough page connector B to block 424, which is described below. If,however, the master 284 determines at block 410 that the attempt toclaim coherence ownership is successful, master 284 performs anycoherence “cleanup” necessary to ensure that it alone has a valid cachedcopy of the target cache line, as shown at block 412. The coherence“cleanup” typically entails issuing one or more kill requests on localinterconnect 114 and/or system interconnect 110 via interconnect logic212 to invalidate other cached copies of the target memory block.

Next, at block 414 master 284 of L3 cache 232 updates the coherencestate of the target memory block in cache directory 292 of L3 cache 232in accordance with Table V, below.

TABLE V Initial Final L3 State L3 State M I Mu I Me I T I Te I Tn I TenI SL I S I Ig n/a In n/a I n/a

As illustrated at block 416, master 284 of L2 cache 230 also updates thestate of the target memory block in cache directory 292 of L2 cache 230in accordance with Table VI, below. As indicated, the target memoryblock will have an M or Mu coherency state, depending upon whethersharing of the target memory block should be encouraged. Thisdetermination can be made on a number of factors, including the type ofstore access that updated the target memory block. Further details canbe found, for example, in U.S. Pat. No. 6,345,343 and U.S. patentapplication Ser. No. 11/423,717, which are incorporated herein byreference.

TABLE VI Initial L2 or L3 State Final L2 State M M or Mu Mu M or Mu Me Mor Mu T M or Mu Te M or Mu Tn M or Mu Ten M or Mu Sl M or Mu S M or MuIg n/a In n/a I n/aThe process proceeds from block 416 to block 430, which is describedbelow.

Referring now to block 424, master 284 of L2 cache 230 requests thetarget memory block and permission to modify the target memory blockfrom the distributed system coherence manager by transmitting anappropriate command (e.g., Read-with-intent-to-modify (RWITM)) to thelocal instance of interconnect logic 212. Master 284 then updates thecoherence state for the target memory block in its cache directory 292in accordance with the coherence response for its request (block 426).Assuming the request was successful, master 284 of L2 cache 230 mergesthe store data 249 received from processor core 202 with the targetmemory block (block 430). Thus, master 284 may update one or moregranules 290 of the target memory block. In addition, once eviction ofthe L2 victim is complete, master 284 of L2 cache 230 updates cachearray 284 with the target memory block (block 432). Thereafter, theprocess ends at block 434.

With reference now to FIG. 5, there is illustrated a high levelflowchart of exemplary method of evicting a victim cache line from an L2cache and casting-in the victim cache line into an L3 victim cache inaccordance with the prior art. The steps depicted on the left side ofFIG. 5 are those performed by a conventional L2 cache and those shown onthe right side of FIG. 5 are performed by a conventional L3 victimcache. Steps are generally shown in chronological order, with timeadvancing in the direction of arrow 500.

The illustrated process begins at block 502 in response a core load orcore store request missing in the L2 cache. In response to the L2 cachemiss, the L2 cache selects a victim line for replacement in accordancewith a selected replacement policy (e.g., least recently used), asdepicted at block 504. As indicated at block 506, the L2 cache thenreads the L2 directory to determine whether or not a castout should beperformed, for example, by determining if the selected victim line has adata-valid coherence state (e.g., Mx, Tx or Sx, where the “x” refers toany variant of the base coherence state) or a scope-state indicationcoherence state, such as Ig or In. If not, then the CO buffer allocatedto the L2 eviction is deallocated and assumes the “done” state (block510). Because the victim line contains no valid data that must bepreserved, the L2 cache can also indicate that the storage location ofthe victim cache line in the L2 array has been evacuated (blocks 512,520) and can be filled with a new cache line of data.

Returning to block 506, if the L2 cache determines that the L2 directoryindicates that a castout is to be performed, then the L2 cache reads thevictim cache line from the L2 cache array into a castout (CO) buffer, asdepicted at block 508. The L2 cache then indicates that the storagelocation of the victim cache line in the L2 array has been evacuated(blocks 512, 520). In addition, the L2 cache transmits a cast-in commandto the L3 victim cache (block 514). The cast-in command may contain orbe accompanied by the real address of the victim cache line, the L2coherence state, and a CO buffer ID.

In response to receipt of the cast-in command, the L3 victim cache readsthe coherence state associated with the specified address in itsdirectory (block 530). If the L3 directory indicates a data-validcoherence state (block 532), then the process proceeds to block 542,which is described below. If, however, the L3 victim cache determinesthat the address specified by the cast-in command misses in the L3 cachearray, then the L3 victim cache begins the process of evicting aselected victim cache line from the L3 array (block 540), as shown inFIG. 7A. The L3 victim cache then initiates a data move of the cast-indata from the CO buffer of the L2 cache to the L3 victim cache byissuing to the L2 cache a data move command specifying the relevant CObuffer ID (block 542). In addition, the L3 victim cache updates the L3directory in accordance with Table VII below (block 544). (Thedesignation Err in Table VII indicates an error condition.) Followingblock 544, the L3 directory update is complete, as depicted at block546.

TABLE VII Initial L3 L2 Castout State State M Mu Me T Te Tn Ten SL S IgIn M Err Err Err Err Err Err Err Err Err M M Mu Err Err Err Err Err ErrErr Err Err Mu Mu Me Err Err Err Err Err Err Err Err Err Me Me T Err ErrErr Err Err Err Err T T T T Te Err Err Err Err Err Err Err Te Te Te TeTn Err Err Err Err Err Err Err Tn Tn Tn Tn Ten Err Err Err Err Err ErrErr Ten Ten Ten Ten SL Err Err Err T Te Tn Ten Err SL Ig SL S Err ErrErr T Te Tn Ten SL S Ig S Ig M Mu Me T Te Tn Ten Ig Ig Ig Ig In M Mu MeT Te Tn Ten SL S Ig In I M Mu Me T Te Tn Ten SL S Ig In

In response to receipt of the data move command of the L3 victim cacheissued at block 542, the L2 cache transfers the data in the specified CObuffer to the L3 victim cache, as indicated at block 550. In a typicalimplementation, the victim cache line is transmitted in association withthe CO buffer ID. Following the data transfer, the L2 cache retires ordeallocates the CO buffer (block 552), indicating usage of the CO bufferis complete (block 554). In response to receipt of the victim cache lineand CO buffer ID, the L3 victim cache places the cast-in data into theL3 array in the location indicated by the CO buffer ID (block 560),thereby completing the movement of the victim cache line from the L2cache to the cache array of the L3 victim cache (block 562).

It should be appreciated that the conventional process depicted in FIG.5 consumes power when the L2 cache array is read at block 508,regardless of whether or not a cast-in to the L3 victim cache isperformed. Power in also dissipated in transferring cast-in data fromthe L2 cache to the L3 victim cache, even in the case of an L3 cache hitin which no data transfer is necessary. To reduce power dissipation, atleast one embodiment reduces data movement and/or array accesses in acache hierarchy including an upper level cache and a lower level victimcache. An exemplary process for performing an L2-to-L3 cast-in havingreduced power consumption is now described with reference to FIG. 6.

Referring now to FIG. 6, there is depicted a high level flowchart of aprocess of performing an L2 eviction and casting-in the victim cacheline into an L3 victim cache in accordance with the one embodiment. Aswith FIG. 5, the steps depicted on the left side of FIG. 6 are thoseperformed by an L2 cache, such as L2 cache 230, and those shown on theright side of FIG. 6 are performed by an L3 victim cache, such as L3cache 232. Steps are generally shown in chronological order, with timeadvancing in the direction of arrow 600.

The illustrated process begins at block 602 in response to an L2 cachemiss as shown, for example, at block 310 of FIG. 3 or block 410 of FIG.4. In response to the L2 cache miss, L2 cache 230 allocates a CO buffer295 to perform an L2 eviction and selects a victim cache line forreplacement in accordance with a selected replacement policy (e.g.,least recently used or a variant thereof), as shown at block 604. Asindicated at block 606, L2 cache 230 (i.e., master 284 of L2 cache 230)then reads cache directory 292 of L2 cache 230 to determine whether ornot a castout is to be performed, for example, by determining if theselected victim cache line has a data-valid coherence state (e.g., Mx,Tx or Sx, where the “x” refers to any variant of the base coherencestate) or a scope-state indication coherence state, such as Ig or In. Ifnot, then the CO buffer 295 allocated to the L2 eviction is deallocatedand assumes the “done” state (block 610). Because the victim cache linecontains no valid data that must be preserved, L2 cache 230 can alsoindicate that the storage location of the victim cache line in the L2cache array 284 has been evacuated (blocks 612, 614) and can be filledwith a new cache line of data (i.e., the target cache line of therequest of the processor core).

Returning to block 606, if the L2 cache determines that L2 cachedirectory 292 indicates that a castout is to be performed, L2 cache 230does not immediately perform a read of L2 cache array 284, as isperformed in the conventional process depicted in FIG. 5. Instead, L2cache 230 transmits a cast-in command to the L3 cache 232 (block 608).The cast-in command may contain or be accompanied by the real address ofthe victim cache line, the L2 coherence state, and the CO buffer ID ofthe allocated CO buffer 295.

In response to receipt of the cast-in command, L3 cache 232 reads thecoherence state associated with the specified address in its L3 cachedirectory 292 (block 620). If the L3 cache directory 292 indicates adata-valid coherence state (block 622), then the cast-in data alreadyresides in the L3 cache array 284, and no data update to the L3 cachearray 284 is required, as indicated by block 624. Accordingly, L3 cache232 signals L2 cache 230 to retire the CO buffer 295 allocated to the L2eviction by issuing an appropriate command specifying the CO buffer ID,as indicated by the arrow connecting block 622 to block 640. Inaddition, as shown at block 630, L3 cache 232 updates the coherencystate of the victim cache line in the L3 cache directory 292 inaccordance with Table VII, above. Thereafter, the L3 directory updatecompletes at block 632.

Referring again to block 622, if L3 cache 232 determines that theaddress specified by the cast-in command misses in L3 cache array 284,then L3 cache 232 begins the process of evicting a selected victim cacheline from L3 cache array 284 (block 626), as described further belowwith reference to FIG. 7A. L3 cache 232 then provides to L2 cache 230 astatus signal referencing the CO buffer ID, thereby indicating that adata move from L2 cache 230 to L3 cache 232 will be performed (block628). In addition, as shown at block 630, L3 cache 232 updates thecoherency state of the victim cache line in L3 cache directory 292 inaccordance with Table VII, above. Thereafter, the L3 directory updatecompletes at block 632. Thus, the directory update can be performed inadvance of initiation of the data move.

Referring now to block 642, in response to receipt of the status signalfrom L3 cache 232 indicating that a data move is to be performed, L2cache 230 expends the power required to read the selected victim cacheline from the L2 cache array 284 into the allocated CO buffer 295. Inresponse to the read of L2 cache array 284, L2 cache 230 can indicatethat the storage location of the victim cache line in the L2 array hasbeen evacuated (blocks 644, 646) and can therefore be filled with a newcache line of data. In addition, L2 cache 230 sends to L3 cache 232 adata ready signal specifying the CO buffer ID in order to indicate thatthe victim cache line has been read into the allocated CO buffer 295(block 650).

In response to the data ready signal, L3 cache 232 initiates a data moveof the cast-in data from the CO buffer 295 of L2 cache 230 to L3 cache232 by issuing to L2 cache 230 a data move command specifying therelevant CO buffer ID (block 652). In response to receipt of the datamove command of L3 cache 232, L2 cache 230 transfers the data in thespecified CO buffer 295 to L3 cache 232, as indicated at block 654. In atypical implementation, the victim cache line is transmitted inassociation with the CO buffer ID. Following the data transfer, L2 cache230 retires or deallocates the CO buffer 295 allocated to the L2eviction (block 656), indicating usage of the CO buffer 295 is complete(block 658). In response to receipt of the victim cache line and CObuffer ID, L3 cache 232 places the cast-in data into L3 cache array 284in the location indicated by the CO buffer ID (block 660), therebycompleting the movement of the victim cache line from L2 cache 230 tothe cache array of the L3 cache 232 (block 662).

With reference now to FIG. 7A, there is illustrated a high level logicalflowchart of an L3 eviction in accordance with one embodiment. Theprocess begins at block 700, for example, in response to initiation ofan L3 eviction at block 626 of FIG. 6. In response to initiation of theL3 eviction, L3 cache 232 (i.e., master 284 of L3 cache 232) allocates aCO buffer 295 to the L3 eviction and selects a victim cache line forreplacement in accordance with a selected replacement policy (e.g.,least recently used), as shown at block 702. As indicated at block 704,L3 cache 232 then reads the L3 cache directory 292 to determine whetheror not the selected victim cache line has an associated coherence statethat indicates a castout should be performed. In at least oneembodiment, the determination is made in accordance with Tables I and IIabove.

If L3 cache 232 determines at block 704 that no castout is to beperformed, then the CO buffer 295 allocated to the L3 eviction isdeallocated and assumes the “done” state (block 710). Because the victimcache line contains no valid data that must be preserved, the L3 victimcache can also indicate that the storage location of the victim cacheline in the L3 array has been evacuated (blocks 712, 714) and can befilled with a new cache line of data.

Returning to block 704, if the L3 victim cache determines that a castoutis to be performed for the victim cache line, then the L3 cache readsthe victim cache line from the L3 cache array into the allocated castout(CO) buffer, as depicted at block 706. The L3 cache then indicates thatthe storage location of the victim cache line in the L3 array has beenevacuated (blocks 712, 714). In addition, the L3 victim cache transmitsa CO command 270 on the interconnect fabric via interconnect logic 212(block 716) and then awaits a combined response (from the process shownin FIG. 7C) providing a system-wide coherency response to the CO command(block 720). The activity of the snoopers (e.g., IMCs 206 and snoopers286 of L2 caches 230 and L3 caches 232) in response to receipt of the COcommand and the generation of the combined response are described belowwith reference to FIGS. 7B and 7C, respectively.

In response to receipt of the combined response of the CO command, theL3 victim cache determines whether or not the combined responseindicates success of the CO command at block 722. If not, the L3 victimcache waits for a “backoff” time, which can be selected randomly withina predetermined range in order to reduce deadlocks (block 724).Thereafter, the process returns to block 716, which has been described.Referring again to block 722, if the combined response indicates thatthe CO command was successful, the L3 victim cache determines at block726 whether the castout entails transmission of the victim cache line.For example, if the victim cache line is in the Ig state, meaning thatthe data is invalid, then no transmission of the data of the victimcache line is to be performed. If, on the other hand, the victim cacheline is in the T state, the L3 victim cache will determine that thevictim cache line data are to be transmitted to a snooper. If adetermination is made that the victim cache line data are to betransmitted, the L3 victim cache transmits the victim cache line data274 from the CO buffer to the destination (e.g., an IMC 206) at block728. Thereafter, the L3 victim cache retires the CO buffer allocated tothe L3 eviction (block 730), giving the CO buffer a “done” status (block732). If, however, the L3 victim cache determines at block 726 that notransmission of the victim cache line data is to be performed, then theprocess simply passes from block 726 to blocks 730 and 732, which havebeen described.

Referring now to FIG. 7B, there is depicted a high level logicalflowchart of an exemplary process by which each snooper (e.g., IMC 206or snooper 286 of an L2 cache 230 or L3 cache 232) receiving a castout(CO) command handles the CO command in accordance with one embodiment.The process begins at block 740 of FIG. 7B and then proceeds to block742, which illustrates that if the snooper receiving the CO command isnot a memory controller, such as an IMC 206, then the snooper provides aNull partial response to the CO command (block 744). The Null partialresponse indicates that the snooper has no interest in the command orits outcome. If the snooper of the CO command is a memory controller,then the process passes from block 742 to block 746. Block 746illustrates the memory controller determining whether or not it isassigned the real address specified by the CO command. If not, thememory controller issues a Null partial response (block 748).

If, however, the snooping memory controller determines at block 746 thatit is assigned the real address specified by the CO command, then thememory controller determines at block 750 whether or not it hassufficient resources (e.g., a queue entry and an available access cycle)available to currently handle the CO command. If not, the memorycontroller provides a Retry partial response requesting that the COcommand be retried (block 752). If, on the other hand, the snoopingmemory controller determines that it has sufficient resources currentlyavailable to handle the CO command, then the snooping memory controllerdetermines at block 754 whether or not the real address specified by theCO command collides with the address of a pending, previously receivedcommand. If so, then the snooping memory controller provides a Retrypartial response requesting that the CO command be retried (block 756).

If the snooping memory controller does not detect an address collisionat block 754, then the snooping memory controller allocates resource(s)for handling the CO command (block 758) and provides an Ack partialresponse (block 760), which acknowledges receipt of the CO command by aninterested snooper. Thereafter, the snooping memory controller awaitsreceipt of the combined response (CRESP) generated by the process ofFIG. 7C for the CO command (block 762). In response to receipt of thecombined response of the CO command, the snooping memory controllerdetermines whether or not the combined response indicates success of theCO command at block 764. If not, the snooping memory controller retiresthe resource(s) allocated to the CO command (block 766), and the processends at block 768.

Referring again to block 764, if the combined response indicates thatthe CO command was successful, the snooping memory controller determinesat block 770 whether the combined response indicates that the castoutentails transmission of the victim cache line to the snooper. If not,the process proceeds to block 774, which is described below. If,however, the combined response indicates that the castout entailstransmission of the victim cache line to the snooper, the snoopingmemory controller awaits receipt of the victim cache line data at block772. Thereafter, at block 774, the snooping memory controller updatessystem memory 108 with control information (e.g., the scope informationrepresented by certain of the coherence states) and the victim cacheline data, if any. Thereafter, the process passes to block 766 and 768,which have been described.

With reference now to FIG. 7C is a high level logical flowchart of anexemplary process by which the partial responses of a castout (CO)command are utilized to generate a combined response in accordance withone embodiment. The illustrated process may be performed, for example,by a predetermined instance of coherence management logic 210, such asthe instance of coherence management logic 210 located in the processingunit 104 of the L3 cache 232 initiating the castout.

The illustrated process begins at block 780 in response to receipt bycoherence management logic 210 of a partial response of a snooper to aCO command of an L3 cache 232 and then proceeds to block 782. Block 782depicts coherence management logic 210 logging the partial response ofthe CO command and waiting until all such partial responses have beenreceived and logged. Coherence management logic 210 next determines atblock 784 whether any of the partial responses were Retry partialresponses. If so, coherence management logic 210 generates and providesto all participants a Retry combined response (block 786). If none ofthe partial responses were Retry partial responses, then coherencemanagement logic 210 provides a Success combined response if the partialresponses include an Ack partial response (blocks 788 and 790).

If no Retry or Ack partial response was received for the CO command,coherence management logic 210 determines at block 792 whether the COcommand was issued on the interconnect fabric with a global scopeincluding all processing nodes 102. If so, the process ends with anerror condition at block 794 in that no memory controller responded tothe CO command as responsible for the real address specified by the COcommand. If, however, coherence management logic 210 determines at block792 that the CO command was issued with a more restricted scope than aglobal scope including all processing nodes 102, then coherencemanagement logic 210 generates and provides to all participants a RetryGlobal combined response indicating that the L3 cache 232 that issuedthe CO command should retry the CO command with a global scope includingall processing nodes 102 of data processing system 100.

As has been described, a processing unit for a multiprocessor dataprocessing system includes a processor core and a cache hierarchycoupled to the processor core to provide low latency data access. Thecache hierarchy includes an upper level cache coupled to the processorcore and a lower level victim cache coupled to the upper level cache. Inresponse to a prefetch request of the processor core that misses in theupper level cache, the lower level victim cache determines whether theprefetch request misses in the directory of the lower level victim cacheand, if so, allocates a state machine in the lower level victim cachethat services the prefetch request by issuing the prefetch request to atleast one other processing unit of the multiprocessor data processingsystem. Because at least some prefetches are handled by state machinesin the lower level victim cache, the number of state machines in theupper level cache needed for a balanced system can be greatly reduced.

While one or more embodiments have been particularly shown anddescribed, it will be understood by those skilled in the art thatvarious changes in form and detail may be made therein without departingfrom the spirit and scope of the invention. For example, althoughaspects of the present invention have been described with respect todata processing system hardware, it should be understood that one ormore embodiments of the present invention may alternatively beimplemented as a program product for use with a data processing system.Such program product(s) include(s) a computer readable medium thatstores or encodes program code that directs the functions of the presentinvention. The computer readable medium may be implemented, for example,as a tangible storage medium (e.g., CD-ROM, DVD, diskette or hard disk,system memory, flash memory, etc.) or communication media, such asdigital and analog networks.

As an example, the program product may include data and/or instructionsthat when executed or otherwise processed on a data processing systemgenerate a logically, structurally, or otherwise functionally equivalentrepresentation (including a simulation model) of hardware components,circuits, devices, or systems disclosed herein. Such data and/orinstructions may include hardware-description language (HDL) designentities or other data structures conforming to and/or compatible withlower-level HDL design languages such as Verilog and VHDL, and/or higherlevel design languages such as C or C++. Furthermore, the data and/orinstructions may also employ a data format used for the exchange oflayout data of integrated circuits and/or symbolic data format (e.g.information stored in a GDSII (GDS2), GL1, OASIS, map files, or anyother suitable format for storing such design data structures).

1. A processing unit for a multiprocessor data processing system, saidprocessing unit comprising: a processor core; and a cache hierarchycoupled to the processor core to provide low latency data access, thecache hierarchy including an upper level cache coupled to the processorcore and a lower level victim cache coupled to and populated by dataevicted from the upper level cache, each of the upper level cache andthe lower level victim cache including a respective cache directory anda respective data array, wherein responsive to a prefetch request of theprocessor core that misses in the upper level cache, the lower levelvictim cache determines whether the prefetch request misses in thedirectory of the lower level victim cache and, if so, allocates a statemachine in the lower level victim cache that services the prefetchrequest by issuing the prefetch request to at least one other processingunit of the multiprocessor data processing system; wherein: the prefetchrequest is a leading prefetch request; the processor core includes astreaming prefetcher that generates the leading prefetch request and atrailing prefetch request both targeting the target memory block; thelower level victim cache, responsive to receipt of the trailing prefetchrequest, provides the target memory block to the upper level cache,preserves the target memory block in the lower level victim cache in ashared coherence state, and updates a replacement order of the targetmemory block to a position other than most recently used.
 2. Theprocessing unit of claim 1, wherein: the upper level cache is a leveltwo (L2) cache; and the lower level victim cache is a level three (L3)victim cache.
 3. The processing unit of claim 1, wherein the statemachine in the lower level cache is a prefetch machine dedicated tomanaging servicing of prefetch requests.
 4. The processing unit of claim1, wherein the processing unit, responsive to receiving a target memoryblock requested by the prefetch request, installs the target memoryblock in the data array of the lower level victim cache but not in thedata array of the upper level cache.
 5. A data processing system,comprising: at least one system memory; and a plurality of processingunits coupled to the system memory, wherein a processing unit among theplurality of processing units includes: a processor core; and a cachehierarchy coupled to the processor core to provide low latency dataaccess, the cache hierarchy including an upper level cache coupled tothe processor core and a lower level victim cache coupled to andpopulated by data evicted from the upper level cache, each of the upperlevel cache and the lower level victim cache including a respectivecache directory and a respective data array, wherein responsive to aprefetch request of the processor core that misses in the upper levelcache, the lower level victim cache determines whether the prefetchrequest misses in the directory of the lower level victim cache and, ifso, allocates a state machine in the lower level victim cache thatservices the prefetch request by issuing the prefetch request to atleast one other processing unit of the data processing system; wherein:the prefetch request is a leading prefetch request; the processor coreincludes a streaming prefetcher that generates the leading prefetchrequest and a trailing prefetch request both targeting the target memoryblock; the lower level victim cache, responsive to receipt of thetrailing prefetch request, provides the target memory block to the upperlevel cache, preserves the target memory block in the lower level victimcache in a shared coherence state, and updates a replacement order ofthe target memory block to a position other than most recently used. 6.The data processing system of claim 5, wherein: the upper level cache isa level two (L2) cache; and the lower level victim cache is a levelthree (L3) victim cache.
 7. The data processing system of claim 5,wherein the state machine in the lower level cache is a prefetch machinededicated to managing servicing of prefetch requests.
 8. The dataprocessing system of claim 5, wherein the processing unit, responsive toreceiving a target memory block requested by the prefetch request,installs the target memory block in the data array of the lower levelvictim cache but not in the data array of the upper level cache.
 9. Amethod of data processing in a multiprocessor data processing systemcontaining a processing unit including a processor core and a cachehierarchy coupled to the processor core to provide low latency dataaccess, wherein the cache hierarchy includes an upper level cachecoupled to the processor core and a lower level victim cache coupled toand populated by data evicted from the upper level cache, each of theupper level cache and the lower level victim cache including arespective cache directory and a respective data array, said methodcomprising: a streaming prefetcher in the processor core generating aleading prefetch request and a subsequent trailing prefetch request bothtargeting a target memory block; the lower level victim cache receivingthe leading prefetch request of the processor core after the leadingprefetch request misses in the upper level cache; in response toreceiving the leading prefetch request, the lower level victim cachedetermining whether the prefetch request misses in the directory of thelower level victim cache; if a determination is made that the leadingprefetch request misses in the directory of the lower level victimcache, allocating a state machine in the lower level victim cache thatservices the leading prefetch request by issuing the leading prefetchrequest to at least one other processing unit of the multiprocessor dataprocessing system; and the lower level victim cache, responsive toreceipt of the trailing prefetch request, providing the target memoryblock to the upper level cache and preserving the target memory block inthe lower level victim cache in a shared coherence state.
 10. The methodof claim 9, wherein: the upper level cache is a level two (L2) cache;the lower level victim cache is a level three (L3) victim cache; andsaid receiving step comprises receiving the prefetch request at the L3victim cache.
 11. The method of claim 9, wherein: the state machine inthe lower level cache is a prefetch machine dedicated to managingservicing of prefetch requests; and the method further comprisesallocating the prefetch machine from a pool containing a plurality ofprefetch machines.
 12. The method of claim 9, said method furthercomprising: in response to receiving a target memory block requested bythe prefetch request, installing the target memory block in the dataarray of the lower level victim cache but not in the data array of theupper level cache.